The general aim of this study is to gain a better understanding of a phenomena that we have observed in a large minority (about 40%) of non-demented patients with Parkinson's disease. The phenomena involves PD patients showing a greater degree of semantic facilitation (priming) than controls on a version of the lexical decision task used in our laboratory. Our general strategy in studying this phenomena will be to determine whether this counterintuitive effect can be related to any of the other cognitive and motor abnormalities present in non-demented PD patients. Three potential explanations for increased semantic priming in PD will be considered: the motor slowing facilitation hypotheses, the degraded semantic knowledge hypothesis, and the impaired decision making hypothesis. The first experiment will seek to determine whether motor slowing and reduced recognition vocabulary contribute to semantic facilitation on a word pronunciation task, where postlexical decision making processes contribute little to performance. This experiment will compare the performance of Parkinson patients on and off anti-Parkinsonian medication. The second experiment will manipulate the type of prime (associative vs categorical) and the base rates for words appearing on the lexical decision task in order to influence the decision making strategy of subjects. The third experiment will attempt to determine whether increased semantic priming is greatest for words about which subjects have degraded semantic knowledge. This study will attempt to replicate previous work in Alzheimer's disease and extend the results to demented and nondemented patients with Parkinsonism. Furthermore, this study will aim to determine whether the breakdown of semantic knowledge, when it is observed in dementing conditions, involves specific words or specific case relations. Data from this study may help clarify inconsistent findings in the literature about lexical decision making in Parkinsonism. They may also establish the importance of the neglected decision making stage of lexical decision performance and help with the interpretation of lexical decision results obtained in other aphasic and degenerative conditions. These experiments may contribute to the development of a neuropharmacologic theory of cognitive functioning in Parkinsonism and may identify a subgroup of nondemented Parkinson patients at risk for cognitive dysfunction.